Powder coating techniques are now extensively used in industry in connection with a Wide diversity of workpieces. A persistent and virtually universal problem associated with such operations involves the effective handling of unused powder (particulate coating material), particularly when coating occurs from fluidized bed or spraying operations because of the natural tendency for the relatively fine particles of powder to drift. This creates an annoyance and hazard to personnel present in the vicinity, and it increases operating costs due to waste of the coating material.
Systems have therefore been developed for controlling fugitive powder. They usually employ vacuum-operated collecting tanks, into Which powder may be withdrawn from the coating area to provide a reservoir from Which recycle may be had. Many of these systems are effective and quite satisfactory.
Virgin powder must of course be introduced into any such coating system to replenish that Which has either been deposited upon the workpiece or depleted through unavoidable losses. It Will be appreciated that the makeup of powder taken from the coating operations will vary from that freshly supplied, and that adding a quantity of virgin powder to powder that has been recycled and collected Will (if not otherwise mixed) result in distinct strata Within the supply; coatings produced therewith would tend to lack desirable (or indeed, essential) uniformity.
Moreover, for the achievement of optimal coatings it is important that the powder be of low moisture content, usually requiring that it be dried prior to introduction into the coating unit. Dryness is particularly important When deposition is achieved by electrostatic attraction, as for example by the method taught in Dunford et al U.S. Pat. No. 4,606,928, and in other patents designated therein.
A very effective system satisfying many of the forgoing needs is described in Carlson U.S. Pat. No. 4,123,175. However, the Carlson system includes no means for drying of the powder, and normally a separate drying chamber (interposed between the collection unit and the coating apparatus) is employed therewith; such a dryer may function by utilizing injected air to fluidize the powder supported upon a porous plate. Primary among the disadvantages of an arrangement of the kind are the time lags that occur both in commencement of the supply of powder to the coating chamber, When it is called for, and also in termination of the flow after the demand has been satisfied.
Accordingly, the main objects of the present invention are to provide novel apparatus for collecting powder used for coating workpieces, which apparatus inherently dries the powder and ensures the uniformity of the powder blend that is withdrawn therefrom, and to provide a novel coating system utilizing such apparatus.
Other objects of the invention are to provide an apparatus and system having the foregoing features and advantages, which are also of relatively simple and inexpensive construction, and which are efficient in operation and convenient to use.